In our prospective study of bone mineral density (BMD) in lactating women, average bone mineral loss is more than 1% per month for each month of lactation. That study, funded as a FIRST Award, included a research plan whereby urine and serum were secured and stored with the stated intent to seek additional funding for hormone analyses in the event that important bone mineral changes were observed. In view of the striking bone loss observed, as well as the observation of recovery at the spine, we request funding to conduct laboratory analysis of serum and urine and to relate those laboratory findings to those data already collected. This provides a unique model to characterize markers of loss and recovery. More than 115 women have been recruited for participation. Data already collected includes BMD measurement by dual energy X-ray densitometry of the proximal femur and lumbar spine at 2 weeks, 2 months, 4 months, 6 months, 12 months and 18 months after parturition. At each time point, information about diet, physical activity, use of medications, and infant feeding practices was collected and blood and urine samples secured. The goals for this study are: 1. To analyze urine samples for indices of bone turnover and urinary mineral excretion (calcium, phosphate and creatinine) and relate these data to bone loss and recovery. 2. To analyze collected sera for parathyroid hormone (PTH), prolactin, estradiol, and 1,25-dihydroxyvitamin D and relate these data to bone loss and recovery. 3. To propose and test mechanisms that might explain rapid BMD loss with lactation and potential remineralization. The major statistical approaches will be analysis of variance, analysis of covariance, growth curve analyses. Because peak bone mass is believed important in providing sufficient bone mineral reserve so that bone loss associated with menopause and aging does not leave the individual at risk for osteoporotic fracture, this bone loss from lactation is important. These unique and already-collected data, coupled with newly generated data, offer an extraordinary and efficient opportunity to elucidate mechanisms(s) and models for bone loss and potential remineralization.